did not cave in upon the iron
lining for several hours after the shield had passed, sometimes not for
a day or more, which permitted the space between it and the iron to be
grouted. The fine gray or beach sand and the quicksand closed in almost
at once. The quicksand has a tendency to fill in under the iron from the
sides and in places to leave a cavity at about the horizontal diameter
which was not filled from above, as the sand, being dried out by the
air, stood up fairly well and did not cave against the iron, except
where nearly horizontal at the top.

The total quantity of grout used on the work was equivalent in set
volume to 249,647 bbl. of 1 to 1 Portland cement grout, of which 233,647
bbl. were ejected through the iron lining, an average of 14.93 bbl. per
lin. ft. The cost of grout ejected outside of the river tunnels was 93
cents per bbl. for labor and $2.77 for "top charges." East of the Long
Island shaft the corresponding costs were $0.68 and $1.63, the
difference being partly due to the large percentages of work done in the
normal air at the latter place.

CAULKING AND LEAKAGE.

Up to August, 1907, the joints between the segments of the cast-iron
lining were caulked with iron filings and sal ammoniac, mixed in the
proportion of 400 to 1 by weight. With the air pressure balancing the
hydrostatic head near the tunnel axis, it was difficult to make the
rust-joint caulking tight below the axis against the opposing water
pressure; this form of caulking was also injured in many places by
water dripping from service pipes attached to the tunnel lining. A few
trials of lead wire caulked cold gave such satisfactory results that it
was adopted as a substitute. Pneumatic hammers were used successfully on
the lead caulking, but were only used to a small extent on the rust
borings, which were mostly hand caulked. Immediately before placing the
concrete lining, all leaks, whether in the rust borings or lead, were
repaired with lead, and the remainder of the groove was filled with 1 to
1 Portland cement mortar, leaving the joints absolutely water-tight at
that time. The subsequent development of small seepages through the
concrete would seem to indicate that the repair work should have been
carried on far enough in advance of the concreting to permit the
detection of secondary leaks which might develop slowly. The average
labor cost chargeable against the caulking was 12 cents per lin. ft., to
which should be added 2